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| Reducing tear-out when trimming the top and back flush https://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=48123 |
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| Author: | James Orr [ Thu Aug 11, 2016 1:13 pm ] |
| Post subject: | Reducing tear-out when trimming the top and back flush |
I'm ready to trim the top and back flush to the rim on my current project. The top is LS redwood, which is very brittle and very prone to tear-out. I'm planning to wipe a few coats of shellac around the perimeter before trimming it flush, and I'm going to score with a gramil when I route the binding channels, but I'd love to know if you have any other tricks up your sleeves for reducing tear-out during flush trimming. Thanks ![[:Y:]](./images/smilies/smiley20.gif "Thumbs Up")
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| Author: | Hesh [ Thu Aug 11, 2016 1:17 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
Yep two things. 1) Climb cuts and 2) Take multiple smaller cuts instead of going for gold all at once. The gramil is a good idea too. |
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| Author: | Tim L [ Thu Aug 11, 2016 1:58 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
I use the drum end of my edge sander to get everything close then I flush cut on my binding routing fixture. Climb cutting is good for finishing up. Just closed up a box last night so I am in the same place as you James. |
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| Author: | DannyV [ Thu Aug 11, 2016 2:51 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
Not that I always do it, but a dedicated fresh bit (or freshly sharpened) always makes things go smoother. Whatca makin' ? |
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| Author: | James Orr [ Thu Aug 11, 2016 3:15 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
Brilliant reminders. Thanks, guys! Danny, it's a small-jumbo type guitar. 15 7/8 lower bout. LS redwood top with koa back and sides. I'm excited to get it bound. The binding scheme will be macassar ebony w/black/koa/black and shell purfling. |
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| Author: | Pmaj7 [ Thu Aug 11, 2016 3:17 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
For $500+ tops, every precaution you can think of is in order! LOL |
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| Author: | klooker [ Thu Aug 11, 2016 3:24 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
Do you have your routing routine down? Which direction to come in at for which part? Only building 1 to 2 per year I always have to stop & think about the routing before I start. |
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| Author: | Bryan Bear [ Thu Aug 11, 2016 3:54 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
klooker wrote: Do you have your routing routine down? Which direction to come in at for which part? Only building 1 to 2 per year I always have to stop & think about the routing before I start. Same here, I don't turn out a lot of guitars. I have the little Stew Mac diagram of the routing directions printed out and stored with my router bit. I look at it every time I get ready to rout just in case. 
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| Author: | Jim Watts [ Thu Aug 11, 2016 4:25 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
Robo-sander |
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| Author: | Joe Beaver [ Thu Aug 11, 2016 4:26 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
What Hesh says And what Kevin and Bryan say. Direction, direction, direction (and small cuts with a good up cut end mill if you can find one) I mostly end up using a carbide, flush cut router bit. |
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| Author: | DennisK [ Thu Aug 11, 2016 4:29 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
A chisel and block plane will quickly and safely get you close. Only the last little bit is time consuming with hand tools, and a router should be able to do that quickly and cleanly. |
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| Author: | doncaparker [ Thu Aug 11, 2016 4:42 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
Make sure the bit is clean of all gunk. It is shocking how quickly that stuff builds up with woods like rosewood. I always cut from fat to skinny: outer edge of lower bout to the waist, then outer edge of lower bout to the butt, then outer edge of upper bout to the waist, then outer edge of upper bout to the neck block. That means a climb cut half the time. |
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| Author: | Tom West [ Thu Aug 11, 2016 4:43 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
klooker wrote: Do you have your routing routine down? Which direction to come in at for which part? Only building 1 to 2 per year I always have to stop & think about the routing before I start. Kevin: Climb mill the whole perimeter..........! That way the cutting edge can't get under anything to split it off. Also the wood ahead of the cut is being pushed toward the body. Tom |
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| Author: | Colin North [ Thu Aug 11, 2016 5:33 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
Tom West wrote: klooker wrote: Do you have your routing routine down? Which direction to come in at for which part? Only building 1 to 2 per year I always have to stop & think about the routing before I start. Kevin: Climb mill the whole perimeter..........! That way the cutting edge can't get under anything to split it off. Also the wood ahead of the cut is being pushed toward the body. Tom Shellac for a start. I like to use a chisel and spokeshaves for most of the trimming. (Make saw cuts in top and back plates in the overhangs at the waist, at the narrowest points) I have to admit I'm probably a convert to climbing cuts all the way, I've had less problems the last few guitars doing this rather than the "SM" method. Sharpened bit every use (trim top first, fresh cutting edge) , clean off any resin build up between runs. (Check that bearing retaining screw is tight of course!) SM downcut rebate bit (6-degree cutting edge downshear) seems better than straight flutes. Not much that hasn't been said already. |
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| Author: | Alan Carruth [ Thu Aug 11, 2016 5:59 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
I got a 1/4" spiral cut solid carbide laminate trimmer bit a few years ago that is just great for this. Yes, it was expensive, but considering how much it costs to fix or replace a top with a blowout, it's nothing. |
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| Author: | david farmer [ Thu Aug 11, 2016 6:19 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
I have a trick you might be able to use on the next one. If your using a process that gives you an outline accuracy within your binding/purfling width, you can glue on plates that need no trimming. Some people rabbit the top/back so they are just inside the outline of the sides to facilitate quicker and more even side sanding. If you can glue slightly undersized plates, you don't have to flush trim or cut the rabbit. Just go straight to sanding the sides. |
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| Author: | Terence Kennedy [ Thu Aug 11, 2016 8:43 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
I usually do it upside down on the router table with a top bearing bit. Climb cuts. Hold the sides parallel to the bit. I have tried regular lam trimer and Robo Sander and this works the best for me. I do have to do the final flush up with a sanding block but it is not much. Alan, I had a flush cut spiral bit a while back and the small bearing seemed to score my side. Have you seen that? |
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| Author: | Rod True [ Thu Aug 11, 2016 11:33 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
I remember Mario saying he uses a rotozip guidepoint bit like this. %20r24164v25.jpg) Leaving the top a bit extra wide the rotozip bit just separates the outer scrap on the top from the edge of the body with no chance of tear-out. The guidepoint rides on the guitar side. Did a search fro rotozip and here you go, about 2/3 down. viewtopic.php?f=10101&t=15849&hilit=rotozip happens to be asking about flush cutting redwood too 
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| Author: | kencierp [ Fri Aug 12, 2016 8:59 am ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
We use a 1/4" double bearing guided carbide spiral down-cut (designed to prevent surface chip outs) along with prudent climb cut patterns. |
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| Author: | James Orr [ Fri Aug 12, 2016 11:43 am ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
Great thread. Thanks, everyone! Even just wiping shellac around the edges, the pad wanted to pull fibers up at the slightest catch. I'll be putting these tips into action. - Shellac - A friend's downcut flush trim bit - Climb cuts all the way |
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| Author: | Ken Franklin [ Sat Aug 13, 2016 3:06 am ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
Just use a sharp chisel. |
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| Author: | Woodie G [ Sat Aug 13, 2016 5:47 am ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
One of the shop's boomerang students (back for a second instrument) suggested a change to the build process, which we tried on the last three boxes closed (two dreadnoughts/one 000). Please forgive the bulleted list, but everything - including using the Keurig- is done that way here. 1. Once the top and back are final-fitted (both are already rough-trimmed to 1/4" overhang prior to brace shaping), align the plate on rim and use the same bent pencil tool used to mark the brace overhangs for trimming to accurately transfer the side profile to the underside of the plate. 2. Trim plate to that line with disk and drum sander 3. Goal is a flush fit of edge of plate to rim and elimination of further trimming prior to block-leveling the closed body Time spent on a low-risk activity (marking and sanding to a line) avoids the time spent on a higher-risk activity (routing plate overhangs), so while we found that - while the new process takes longer than knife & chisel trimming as suggested by Mr. Franklin - it takes less in the way of tool handling skills. About 15 minutes more in terms of time spent, but a net benefit if management of project risk is a consideration. |
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| Author: | Clay S. [ Sat Aug 13, 2016 8:09 am ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
"Alan, I had a flush cut spiral bit a while back and the small bearing seemed to score my side. Have you seen that?" Usually that is caused by the bearing spinning with the bit. Oiling the bearing and holding it a little more firmly against the work piece may help eliminate the problem. Also using a 1/2 inch "shear face" router bit will work about as well as a spiral bit and allow you to touch up the edge with a diamond hone if it gets a little dull. |
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| Author: | klooker [ Sat Aug 13, 2016 9:26 am ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
Tom West wrote: Kevin: Climb mill the whole perimeter..........! That way the cutting edge can't get under anything to split it off. Also the wood ahead of the cut is being pushed toward the body. Tom I've gotten better results conventional cutting when the wood grain is oriented properly - much cleaner cuts and less metal & physical stress than dealing with climb cutting. After doing a quick search I see that using a straight bit which is what I've always used was probably the problem. http://www.leevalley.com/us/shopping/Te ... px?p=56809 Kevin Looker |
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| Author: | Mike_P [ Sat Aug 13, 2016 4:14 pm ] |
| Post subject: | Re: Reducing tear-out when trimming the top and back flush |
chatter is an issue with climb cutting, but I've never really had any problems with routes that are taking out minimal material...the technique does take a steady hand and concentration to keep control of things...take it nice and slow! if chatter is a big issue then climb cut with a bigger bearing then clean up with a smaller...I always climb cut first then do a conventional route afterwards. |
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